﴿ ثُمَّ بَدَا لَهُم مِّنۢ بَعْدِ مَا رَأَوُا۟ ٱلْءَايَٰتِ لَيَسْجُنُنَّهُۥ حَتَّىٰ حِينٍۢ ﴾

“For, presently it occurred to the nobleman and his household _ [even] after they had seen all the signs [of Joseph's innocence] - that they might as well imprison him for a time.”

Then it seemed it appeared good to them after they had seen the signs indicating Joseph’s innocence that they should imprison him for a while until such time as people ceased to talk of him. He was thus imprisoned.